This invention relates to analog to digital converters and more particularly to an A/D converter of the stage by stage, successive approximation type.
To interface data processing systems with the real world, an A/D converter is required to perform measurement and a digital to analog converter is required to perform control. A/D converters can be grouped into four classes; namely, feedback, integrating, parallel and cascade. All of these various classes of A/D converters have certain properties which dictate their respective applications.
This invention is concerned with the feedback class of A/D converter and, more particularly, with the successive approximation sub-type. In such a converter, the analog input is successively compared against each weighted bit until the analog input has been approximated by the digital code. Generally, this is a binary code and will require the same number of trials (comparisons) as the number of bits of resolution the converter has; i.e., a 12 bit converter requires 12 successive trials to make a complete conversion. This type of shift-sequenced-feedback successive approximation is well known.
Not as well known is a successive approximation technique instrumented by a succession of identical stages called a stage-by-stage architecture. The key features of this approach are that each cell or stage is identical, and a higher through put rate can be attained. This is because it is not necessary to settle to the same degree of precision at each decision. Each analog stage effectively subtracts half of full-scale from its input and makes a decision via a comparator as to whether the input signal is higher or lower than that reference. The difference is then gain multiplied by a factor of 2, and the absolute value of the amplified difference is passed on to the next stage.
This approach also has an advantage in that the digital logic circuitry required is greatly reduced over the shift-sequenced-feedback architecture. The analog circuitry contained within the blocks, however, is not easily realized in integrated circuit form, and can be a major error source, thereby limiting the accuracy which can be achieved in the converter.
What is needed is an A/D converter of the stage by stage, successive approximation type which can be easily synthesized in integrated circuit form without introducing major error sources and with a minimum number of components required to realize the function.